1. Field of the Invention
The present invention relates to an Fe-Ni alloy sheet for a shadow mask used for a color cathode-ray tube and a method for manufacturing same.
2. Related Art Statement
Along with the recent tendency toward a higher-grade color television set, an Fe-Ni alloy containing nickel of from 34 to 38 wt.% (hereinafter referred to as the "conventional Fe-Ni alloy") is used as an alloy for a shadow mask capable of coping with problems such as a color-phase shift. The conventional Fe-Ni alloy has a far smaller thermal expansion coefficient as compared with a low-carbon steel conventionally applied as a material for a shadow mask.
By manufacturing a shadow mask from the conventional Fe-Ni alloy, therefore, even heating of the shadow mask by an electron beam would hardly cause such problems as a color-phase shift resulting from thermal expansion of the shadow mask.
An alloy sheet for a shadow mask is usually manufactured by the following steps of: preparing an alloy ingot by a continuous casting or by an ingot casting, subjecting the thus prepared alloy ingot to a slabbing rolling, a hot rolling and a cold rolling, thereby manufacturing an alloy sheet.
The alloy sheet for a shadow mask manufactured as above is processed into a shadow mask by the following steps of: forming passage-holes for the electron beam (hereinafter simply referred to as the "holes") in the alloy sheet for a shadow mask by the photoetching (an alloy sheet for a shadow mask as pierced by the etching is hereinafter referred to as the "flat mask"), then subjecting the flat mask to an annealing, then press-forming the annealed flat mask into a curved surface to match with the shape of a cathode-ray tube, and applying a blackening treatment to the surface thereof.
However, the use of the conventional Fe-Ni alloy poses the following problems:
(1) The conventional Fe-Ni alloy, containing nickel in a large quantity, has a strength higher than that of the low-carbon steel. In order to improve press-formability, therefore, a flat mask manufactured from the conventional Fe-Ni alloy must be annealed at a higher temperature than in the case of a flat mask manufactured from the low-carbon steel. Sticking therefore tends to occur in several tens to several hundreds of flat masks made from the conventional Fe-Ni alloy, which are placed one on the top of the other, during the annealing thereof.
(2) In the alloy sheet for a shadow mask manufactured from the conventional Fe-Ni alloy, irregularities may easily occur in the diameter and the shape of the holes pierced by the etching as a result of the segregation of components, as compared with the sheet for a shadow mask manufactured from the low-carbon steel. Irregularities in the hole diameter and the hole shape seriously impair the quality of a color cathode-ray tube.
(3) In a shadow mask manufactured from the conventional Fe-Ni alloy, when the shadow mask is heated by the radiation of an electron beam during the operation of the color cathode-ray tube, gases tend to be produced from the surface of the shadow mask. The production of gases from the surface of the shadow mask seriously impairs the quality of a color cathode-ray tube.
(4) Because of the very low hot workability, the conventional Fe-Ni alloy is susceptible to flaws during a slabbing rolling and a hot rolling, this requiring a large amount of scarfing, leading to a very low production yield.
For the purpose of solving the above-mentioned problems, the following prior arts are known:
(a) Japanese Patent Provisional Publication No. 2-170,922 discloses a method which comprises the steps of: applying a soaking treatment, prior to a hot rolling, to a slab prepared by the continuous casting of an Fe-Ni alloy containing nickel of from 30 to 50 wt.%, in a heating furnace capable of controlling the oxygen concentration to a low level, at a temperature within a range of from 1,200.degree. to 1,350.degree. C. for at least one hour to reduce segregation of nickel and manganese in the slab, thereby inhibiting the production of irregularities in the diameter and the shape of the holes pierced by the etching, caused by a string-like pattern along the rolling direction under the effect of the segregation of the components, and preventing the production of subscale, thus improving the production yield (hereinafter referred to as the "prior art 1").
(b) Japanese Patent Provisional Publication No. 2-182,828 discloses a method which comprises the steps of: heating an ingot of an Fe-Ni alloy containing nickel of from 30 to 80 wt.% and boron of from 0.001 to 0.030 wt.% to a temperature of at least 900.degree. C., forging same with a sectional reduction rate of at least 30% to prepare a slab, and then applying a soaking treatment to the thus prepared slab at a temperature of at least 1,000.degree. C. for at least one hour, thereby inhibiting the production of irregularities in the diameter and the shape of the holes pierced by the etching, caused by a string-like pattern along the rolling direction under the effect of the segregation of the components (hereinafter referred to as the "prior art 2").
The prior arts 1 and 2, while permitting inhibition of the production of irregularities in the diameter and the shape of the holes pierced by the etching, have still the following problems: The surface of each hole pierced by the etching is seriously roughened to present a blurred periphery; sticking of the flat masks during the annealing thereof cannot be prevented; when the shadow mask is heated by the radiation of an electron beam during the operation of the color cathode-ray tube, gases tend to be produced from the surface of the shadow mask; and improvement of the production yield is insufficient.
More specifically, in the prior art 1, although it is possible to inhibit the production of irregularities in the diameter and the shape of the holes pierced by the etching, caused by a string-like pattern along the rolling direction under the effect of the segregation of nickel and manganese, by reducing the segregation of nickel and manganese in the slab through application of the soaking treatment to the slab, silicon segregation cannot be sufficiently reduced. The segregation of silicon in an Fe-Ni alloy remains in the final product more easily than the segregation of nickel and manganese. While the segregation of nickel and manganese is reduced, in the prior art 1, by applying the soaking treatment to the slab as described above, it is impossible in this manner to reduce the segregation of silicon to below a certain level. In the prior art 1, as a result, sticking of the flat masks occurs during the annealing thereof because of the serious segregation of silicon.
With a considerable segregation of silicon, furthermore, the surface of each hole pierced by the etching is seriously roughened to present a blurred periphery, thus producing another etching piercing defect, different from the above-mentioned etching piercing defect caused by the segregation of nickel and manganese, and, as a result, the quality of the color cathode-ray tube is degraded. In addition, since the soaking treatment is applied to the slab at a temperature within a range of from 1,200.degree. to 1,350.degree. C. for at least one hour, the production of surface flaws of the slab caused by the subscale leads to a lower production yield of the slab even when the oxygen concentration in the heating atmosphere is reduced. In the prior art 1, furthermore, because of the presence of fine cracks in the Fe-Ni alloy sheet, a treatment solution such as an etching solution remains in these fine cracks during the etching-piercing. As a result, when the shadow mask is heated by the radiation of an electron beam during the operation of the color cathode-ray tube, gases tend to be produced from the surface of the shadow mask.
The prior art 2 inhibits the production of irregularities in the diameter and the shape of the holes pierced by the etching, caused by a string-like pattern along the rolling direction under the effect of the segregation of nickel and manganese, by inhibiting the segregation of such impurities as carbon, silicon, manganese and chromium onto the crystal grain boundary through addition of boron to the Fe-Ni alloy, and reducing the segregation of components through the forging. As in the prior art 1, however, the segregation of silicon is not sufficiently reduced. In the prior art 2 also, although the segregation of nickel and manganese in the Fe-Ni alloy is reduced by the addition of boron into the Fe-Ni alloy and the application of forging, it is impossible to reduce the segregation of silicon to below a certain level. As in the prior art 1, as a result, sticking of the flat masks occurs during the annealing thereof because of the serious segregation of silicon, and the surface of each hole pierced by the etching is seriously roughened to present a blurred periphery, thus producing an etching piercing defect.
In the prior art 2, furthermore, application of the forging leads to a lower production yield. In addition, in the prior art 2, heating of the shadow mask by the radiation of an electron beam during the operation of the color cathode-ray tube tends to easily produce gases from the surface of the shadow mask, because of the presence of fine cracks in the Fe-Ni alloy sheet, as in the prior art 1. Furthermore, because boron is added to the Fe-Ni alloy in the prior art 2, the segregation of boron onto the crystal grain boundary occurs to a considerable extent, and the surface of each hole pierced by the etching is seriously roughened, thus producing an etching piercing defect similar to that caused by the considerable segregation of silicon, this heavily imparing the quality of the color cathode-ray tube.
Under such circumstances, there is a strong demand for the development of an Fe-Ni alloy sheet for a shadow mask, which is excellent in etching pierceability, ensures the prevention of sticking of the flat masks during the annealing thereof, inhibits the production of gases from the surface of the shadow mask during the operation of the color cathode-ray tube, and gives a high production yield, and a method for manufacturing same, but such an Fe-Ni alloy sheet and a method for manufacturing same have not as yet been proposed.